Lactic acid is widely used as a food ingredient. Lactic acid has in the past been available by fermentation processes utilizing carbohydrates as raw material. In recent years an industrial process has utilized the reaction of hydrogen cyanide with acetaldehyde, followed by the hydrolysis of the resulting lactonitrile employing hydrochloric acid as catalyst. While this route is useful and less costly than the older fermentation process, it will be recognized that there is danger and expense associated with the use of hydrogen cyanide, and an alternate route is therefore desirable. In particular, the lactonitrile route involves cyanide which is highly poisonous and great care must be taken in the manufacturing process to ensure that no cyanide remains in the final product before it can be used as a food ingredient. Also, the process involves highly corrosive chloride which requires expensive materials of construction. A number of possible routes to lactic acid from such fundamental starting materials as olefins, carbon monoxide, water, hydrogen, oxygen, etc. and their reaction products can be visualized, but most such routes are impractical or inoperative. For example, acetaldehyde is available commercially, and carboxylation of acetaldehyde by reaction with carbon monoxide and water has been reported to produce lactic acid (J.Applied Chem., 20, page 7, (1970)), but attempts in our laboratory to use this procedure have resulted in only traces of lactic acid. While the industrial cyanohydrin route to lactic acid has some undesirable cost factors, it is nevertheless an operative, reasonably efficient route which utilizes available raw materials. It follows that for industrial use a new route will need to have fairly high conversions to the desired lactic acid product, or that product along with useful and marketable concomitant products.
Accordingly, it is an object of the present invention to provide a synthetic route to lactic acid which does not require any cyanide and which does not involve corrosive reactants.
It is a further object of the present invention to provide a synthetic process for lactic acid manufacture which requires only cheap, readily available starting materials.
It is a further object of the present invention to provide a process for lactic acid production which involves very simple and inexpensive by-product separation procedures.
It is a further object to provide a route to lactic acid which obtains sufficient yields of desired useful intermediates and by-products and utilizes efficient conversion and separation procedures so as to be technologically and economically practical in terms of cost and efficiency factors with respect to raw materials and conversion procedures.
Hydroformylation processes are well known in the art and have been directed to the production of reaction mixtures comprising substantial amounts of aldehydes and alcohols by the reaction of olefins with carbon monoxide and hydrogen at elevated temperatures and pressures in the presence of certain catalysts. The prior art teaches the use of dicobalt octacarbonyl or its various modified forms as well as carbonyls of other Group VIII metals such as rhodium, ruthenium, and iridium which may also be modified by ligands comprised of organic compounds of Group V elements such as triaryl- and trialkyl-phosphines, arsines, etc. Certain disadvantages are potentially present in the hydroformylation processes described in the prior art art when they are applied to a vinyl ester as the reactant. In particular, if it is desired to synthesize lactic acid by the route described herein, a high selectivity to alpha-acyloxypropionaldehyde in the hydroformylation reaction step is required for a commercially competitive process. Thus, Adkins and Krsek (J Amer. Chem. Soc., 71, 3051 (1949) ) studied the hydroformylation of vinyl acetate with cobalt carbonyl as the catalyst at 125.degree. and under a pressure of 4600 psi of a 1:1 mixture of carbon monoxide and hydrogen. They obtained a 30% yield of alpha-acetoxypropionaldehyde and a 22% yield of beta-acetoxypropionaldehyde. By contrast, the hydroformylation process in accordance with procedures described herein gives yields of over 70% of the alpha-acetoxypropionaldehyde, thus, leading to good yields of lactic acid.